Rotating dynamoelectric machinery, such as motors and generators, whether utilizing or generating alternating current or direct current, have a rotor contact surface, such as commutator or slip rings, fixed to the rotating armature or rotating field, and electrically connected to windings of the rotating armature or rotating field. The windings of the rotating armature or rotating field are connected through brushes or the like which make sliding electrical contact with the rotor contact surface, and are connected to the fixed winding being independently connected to an external power source. A fixed magnetic field may also be provided by permanent magnets.
Brushes are typically made of a carbon particulate, such as graphite, and a binder material, and may also include metallic particles. One or more holes are made in an end of the brush, to accommodate current carrying wires, or pigtails. For higher current operation, angled, intercepting, holes are drilled, and a rivet through the brush is installed at their intersection. The pigtail is looped through the angled holes and around the rivet, and soldered to the rivet. For lower current operation, a hole is made in the end of the brush, the pigtail wire is inserted, and metallic or other conductive particles are tamped in the hole around the pigtail wire to hold it in place.
In order to provide a sliding contact which does not cause significant wear to the rotor contact surface, the brushes are made softer than the rotor contact surface, which is often made of a copper alloy. However, the conducting wires, or pigtails, are also made of copper alloy, and may damage the rotor contact surface after a sufficient amount of the brush wears away. In the rivet style brush, the rivet may contact the rotor contact surface, causing rapid and severe damage.
Many attempts have been made to avoid damage to the rotor contact surface, by providing a signal when a brush wears to a predetermined point, to allow the brush to be moved away from contact with the rotor contact surface when it has worn to a predetermined point. All these attempts have involved modification of a standard brush holder, requiring disassembly of the dynamoelectric machine, and substitution of new brush holding structure. U.S. Pat. No. 942,264, issued to Kimble on Dec. 7, 1909, entitled "BRUSH HOLDER" discloses a brush holder with a spring member which is normally seated in a cavity made in a brush, and is released from the cavity to hold the brush away from a commutator when the brush wears down to the point near the bottom of a cavity. U.S. Pat. No. 1,295,860, issued to Dean, on Mar. 4, 1919, entitled "BRUSH HOLDER", discloses the use of a flexible conductor attached to the pressure exerting device, rather than embedded in a brush, to conduct current, the conducting member being attached to the pressure exerting member, and the pressure exerting member being equipped with an extension which contacted the brush box to prevent the flexible conductor from contacting the brush box. U.S. Pat. No. 2,193,172, issued to Hills, on Mar. 12, 1940, entitled "SAFETY DEVICE FOR GENERATORS", discloses the use of a bracket attached to a cantilevered, pivoting brush holder, the bracket opening a leaf-type switch when the brush wears to a predetermined point, the leaf switch members being formed of different types of metal so as to also open when the generator overheats. U.S. Pat. No. 2,691,114, issued to Lykins, on Oct. 5, 1954, entitled "GENERATOR BRUSH WITH CONDUCTION INDICATOR", discloses the use of a spring-urged clip bearing on the brush, a resilient contact member being provided on, and insulated from, the spring-urged clip, the contact member being engagable with an end of the brush holder upon excess wear of the brush. U.S. Pat. No. 2,813,208, issued to Ritter, on Nov. 12, 1957, entitled "ELECTRICAL CONTACT BRUSH", discloses the use of a spring-biased plunger in a recess in the body of the brush, the plunger being adapted to push the brush away from the commutator surface when the brush wears to the bottom of the recess. U.S. Pat. No. 3,523,288, issued to Thompson, on Aug. 4, 1970, entitled "BRUSH WEAR INDICATOR", discloses the use of a brush having a recess or protuberance thereon, a cantilevered resilient arm attached to the brush holder, which either bears a pin which falls into a recess in the brush when the brush has worn to a predetermined point, or falls off the end of a protuberance on the brush when the brush has worn to a predetermined point, operating an electrical switch. In this manner, although the lead wire is prevented from touching the commutator due to its length, an indication is given to allow replacement of brushes before arcing from the brush to commutator can damage the commutator. U.S. Pat. No. 3,609,429, issued to Thompson, on Sept. 28, 1971, entitled "BRUSH WEAR INDICATOR", discloses the use of a pin falling into a recess in the brush when the brush has worn to a predetermined point, allowing the contact arm to contact the brush holder, and provide a signal that the brush should be replaced. U.S. Pat. No. 3,898,492, issued to Vassos et al., on Aug. 5, 1975, entitled "CURRENT INTERRUPTING BRUSH HOLDER ASSEMBLY", discloses the use of a current carrying trip member which is released from an arm on the brush when the brush wears down to predetermined length, and also discloses the use of a separate contact member sandwiched between a spring and the brush which engages stops on the brush holder, allowing the brush to all away when it has worn to a predetermined length, and further discloses the use of a wedge-shaped contact sandwiched between the spring and the brush, which aligns with a recess in the brush holder when the brush has worn to a predetermined length, and is forced out of contact with the brush by the spring, stopping the motor. U.S. Pat. No. 4,024,525, issued to Baumgartner et al., on May 17, 1977, entitled "BRUSH WEAR INDICATOR", discloses the use of a motor brush having an elongated groove formed in one side, and a brush box having an insulated probe protruding into the groove. As the brush wears to a predetermined point, an end of the groove contacts the insulated probe, providing a signal to indicate that the brush has worn. The movement of the brush may also be restrained, creating the possibility of commutator damage from arching. Also disclosed is a latching warning device connected to the insulated probe. U.S. Pat. No. 4,121,207, issued to Jones, on Oct. 17, 1978, entitled "SWITCH FOR INDICATING BRUSH WEAR", discloses the use of a conventional roller-arm microswitch adjacent a brush box, the roller bearing against a brush through a opening in the brush box. The switch is actuated when the brush has worn to the point where the roller falls off the end of the brush, actuating the switch, and providing an indication that the brush is worn. U.S. Pat. No. 4,172,988, issued to Lowther, on Oct. 30, 1979, entitled "BRUSH WEAR INDICATING MEANS WITH ENGAGABLE ELECTRICAL CONTACTS", discloses the use of a Z-shaped contact element bonded to the top of a brush, the contact element contacting an isulated contact provided on the brush holder when the brush has worn to a predetermined point.
The instant invention provides a brush wear indicating means which avoids the deficiencies and difficulties of previous attempts to provide a signal indicative of brush wear to predetermined length.